1. Field of the Invention
The present invention relates to an OFDM communication apparatus, and more particularly, to an OFDM communication apparatus in a mobile communication system.
2. Description of the Related Art
The main factor causing deterioration of transmission characteristics of ground wave in transmission path is currently multipath interference. An OFDM (Orthogonal Frequency Division Multiplexing) transmission system resistant to the multipath interference has been recently paid attention. The OFDM system multiplexes a plurality of (a few tens to hundreds) digital modulated signals orthogonalized to each other in a signal interval.
A conventional OFDM communication apparatus is explained using FIGS. 1 to 3. FIG. 1 is a partial block diagram illustrating a schematic configuration of the conventional OFDM communication apparatus, FIG. 2 is a schema illustrating a schematic structure of an OFDM transmission/reception signal, and FIG. 3 is a signal space diagram for a multivalue modulated signal.
In the conventional OFDM communication apparatus, a message is quadrature-modulated in modulation section 1, and IFFT (Inverse Fast Fourier Transform) calculated in IFFT section 2 to be an OFDM signal. Meanwhile, a known signal is IFFT calculated in IFFT section 2 to be an OFDM signal. These OFDM signals are D/A converted in D/A conversion section 3 to be a baseband signal. The baseband signal is amplified, and then transmitted through an antenna as a transmission signal.
The received signal received through an antenna is converted into a digital signal in A/D converter 7, FFT (Fast Fourier Transform) calculated in FFT section 6, and subjected to coherent detection in coherent detection section 5 using the pilot symbol assigned for a head of a signal for symbol synchronization acquisition. The coherent detected signal is output to phase compensation section 4, and subjected to phase compensation based on a phase of the pilot symbol.
The received signal contains, as illustrated in FIG. 2, pilot carriers containing the known signal to perform the phase compensation for the received signal other than the pilot symbol. Herein, it is assumed that, as illustrated in FIG. 2, 4 carriers are contained in the received signal. Further, it is assumed that, as illustrated in FIG. 3, the known signal contained in the pilot carrier is transmitted with 2 bits, and that user data is transmitted with 16 QAM (Quadrature Amplitude Modulation) (4 bits).
With respect to the known signal transmitted with the pilot carrier contained in the received signal, a phase difference is detected for each carrier, and the average of phase differences of all the pilot carriers is calculated. This average of phase differences is a phase rotation amount (residual frequency offset correction amount) for the received signal. The rest of the received signal after the pilot carriers are separated, i.e., user data, is subjected to phase compensation in phase compensation section 4 corresponding to the obtained phase rotation amount. Thus, based on the pilot carrier inserted to the transmission signal, the phase rotation amount of the received signal is calculated to detect a phase error.
Usually, thermal noise is superimposed on the received signal. In this case, the thermal noise is superimposed on the pilot carrier and other subcarriers equally. Accordingly, when the phase compensation is performed on the user data based on the known signal transmitted with the pilot carrier, the user data is expected to contain the thermal noise superimposed on the pilot carrier in addition to the thermal noise superimposed on the subcarriers. Therefore, in the communication environment, when the level of the thermal noise is high, in other words, the carrier to noise ratio (C/N ratio) is low, the accuracy of the phase error detection using the known signal deteriorates, resulting in the problem that accurate phase compensation cannot be performed on the user data.
An object of the present invention is to provide an OFDM communication apparatus capable of accurately performing phase compensation on user data even when the carrier to noise ratio (C/N ratio) is low.
The subject matter of the present invention is to perform amplitude adjustment (gain control) of a known signal of a pilot carrier, or assign a signal with a large amplitude in multivalue quadrature amplitude modulation for the pilot carrier, to increase a C/N ratio of the known signal, so that the phase compensation can be performed accurately on the user data even when the C/N ratio is low in the communication environment.